If you're brand new to Windows XP, specifically to customizing it, start with these two articles by Fred Langa:
    - Ten Ways To Make Windows XP Run Better
    - Ten More Ways To Make Windows XP Run Better

You can also visit Langa's System Setup Secrets For Windows XP, Make Windows XP Self-Maintaining, and these more recent ones called Laptop Setup Secrets and Make The Most Of That New PC.

You might also want to look at some of these:
    - The Complete Windows XP Tweak Guide (MortifiedPenguin)
    - The Insider's Guide to Fixing Windows (PC World)
    - Optimizing A Windows Xp PC - Installing, Optimizing And Imaging
    - Customizing Windows XP (Help With Windows)
    - Beginners Guides: 99 Performance Tips for WindowsXP (PC Stats)
    - Top 10 Computer Health Tips (Windows Support Center)
    - Seven Ways to Make Living With Windows Easier (PC World)
    - Windows Rejuvenated! (PC World)
    - Tune Up Your Windows XP Machine for Free (PC World)
    - Keep It Simple ... Cut the clutter on your PC and you can stop aggravations before they begin  (PC World)
    - The Tweaking Experience Registry Guide
    - Optimize Windows XP (ExtremeTech)
    - Setup and Maintenance Tips (Microsoft)
    - How to set performance options in Windows XP (Microsoft)
    - TweakHound's Super XP Tweaking Guide
    - Windows XP Super Tweaks (Black Viper)
    - 20 Windows Tips For Users Old and New (PC Magazine)
    - Windows XP Registry Fixes, Utilities, Illustrations and Tweaks (MVPs.org)
    - Pain-Free Windows Tweaks (PC World)
    - Customizations Tips for Windows (About.com)
    - How to Fix the Biggest PC Annoyances (PC World)
    - 50 Fixes for the Biggest PC Annoyances (PC World)

And if you like, you can carry on at the list of topics at the main page.  Or if you have serious time to kill, try the links at this section.  XP sites, Windows forums, newsletters, and other good things live there.